This invention pertains to the preparation of cellulose ethers having enhanced resistance to enzyme catalyzed hydrolysis and more particularly to the use of a mixture of lithium and sodium hydroxides as the catalyst for the etherification reaction.
Water-soluble cellulose ethers are widely used as thickening agents in a variety of aqueous based systems. However many of these cellulose ethers are subject to enzymatic hydrolysis. The resulting viscosity loss is usually detrimental to the end use properties, as for example, in cellulose ether thickened latex paints. It is known that the enzymatic sensitivity of a cellulose ether is related to, but is probably not an exclusive function of, the percent unsubstituted anhydroglucose units in this cellulose derivative. In turn the unsubstituted anhydroglucose units at a given degree of substitution is expected to be, at least in part, related to the relative reactivity ratios of the various hydroxyl groups in competition for an etherification agent. In agreement with these observations, enzyme resistant hydroxyethyl cellulose produced by the reaction of alkali cellulose with ethylene oxide has been very difficult to produce. The difficulty lies in the formation of the reactive primary alcohol of the hydroxyethyl substituent during the ethoxylation reaction. Chains of --CH.sub.2 CH.sub.2 O-- groups form which compete with the ethoxylation of the cellulose hydroxyl groups, thus leaving many unsubstituted anhydroglucose units.
This problem has prompted several attemps to prepare enzyme resistant cellulose ethers by utilizing, at least in part, an etherification reagent which reduces or eliminates the competitive chaining reaction. For example, U.S. Pat. No. 3,388,082 discloses the preparation of hydroxypropylmethyl cellulose having increased enzymatic resistance. Unfortunately, the latex paint industry has found that good color development and compatability is not possible with hydroxypropylmethyl cellulose and some pigments.
U.S. Pat. Nos. 3,709,876 and 3,769,247 describe a 2-step synthesis of cellulose ethers consisting of cellulose methylation followed by ethoxylation. These products are described as useful as paint thickeners but the 2-step nature of the reaction and formation of large amounts of inorganic halide in the methylation step make the synthesis unattractive from a commercial point of view.